1. Technical Field
The present invention relates to a trigger system for an oscilloscope, and particularly to a system that counts external events to provide a trigger for irregular input signals.
2. Background of the Invention
In conventional digital storage oscilloscopes, a trigger generator receives an input signal and, in response, generates a trigger signal synchronous to when the input signal exceeds a trigger threshold. The trigger generator works together with a time base control circuit which generates a periodic sampling clock signal. The digital oscilloscope samples the input signal in response to each sampling clock signal and stores the sample results in an acquisition memory. Thus, a conventional digital storage oscilloscope essentially duplicates the functionality of an analog oscilloscope; the data is acquired in a regular, periodic fashion.
There are certain data acquisition applications in which it would be more logical to sample the input signal in response to events other than the output of a regular, periodic clock signal. Existing oscilloscopes provide an "external clocking" function that provides just this capability.
By way of example, suppose one wished to display on an oscilloscope the force acting on an axle as a function of the axle's rotational angle. A transducer mounted at an appropriate location on the axle could provide an analog signal corresponding to the force. This analog signal would be the oscilloscope's input signal. If the axle rotates at a constant angular velocity, then an internally-generated, periodic clock signal from a time-base control circuit could clock the sampling of the input signal. Furthermore, another internally-generated periodic clock signal could trigger the oscilloscope. Such an internally-generated clock signal is used in an "auto trigger" mode. However, it is unlikely that the period of this second clock signal would correspond to the rotational period of the axle.
However, if the axle rotates at a non-constant velocity, then a regular, periodic clock signal would not be able to provide samples of the force transducer at regular axle angles. Furthermore, an auto trigger mode would not be appropriate because different rotations of the axle can occur in different amounts of time. Records acquired in an auto trigger mode would not align with each other.
A rotational transducer could be attached to the axle such that it produces a signal every time the axle's angle increases by a certain amount. This external "clock" signal would provide the stimulus for each sampling of the input signal. ("Clock" is in quotes to emphasize that such an externally-generated "clock" signal is not necessarily a regular, periodic signal. It will be used henceforth without quotes, but still with the understanding that an external clock signal is not necessarily a regular, periodic signal.) Another transducer on the axle could indicate when the axle rotates past a reference angle and thus provide the trigger signal.
The just-described data acquisition system requires three transducers: a force transducer, a rotational reference transducer, and a rotational transducer. It would be preferable to provide a data acquisition system that did not require the reference transducer.
What is needed is a system that generates trigger signals for irregularly-timed periodic input signals and that does not require a separate external input to signal the beginning of a period.